In addition to how an espresso machine produces its required brew pressure, the method it uses to heat and maintain the water
at the desired brew temperature distinguishes one design from another. The boiler type affects how the barista operates the
espresso machine, so it's worth understanding how one type differs from another.

Heating Methods

Boiler/Heat Shedding Group: This is the oldest system. Water
is taken directly from the steam boiler at a temperature of roughly
120°C (250°F). The water's temperature drops to brew range in
the group prior to its reaching the coffee. Most spring lever machines
work in this way. Obviously, this is not a very precise way to regulate
temperature. If the group is too cool, the final brew temperature will
be too low; if it overheats, the final brew temperature will be too
high. On commercial lever machines, shots have to be made at just the
right pace to keep the group at the correct temperature; on many home
lever machines, the machine has to be turned off after four or five
shots and left to cool.

Heat Exchanger: Most commercial and larger home machines use
this system. The heat exchanger is basically a pipe inside the boiler.
As the water is pumped to the group, it goes through the pipe and heats
up to brew temperature range. The average temperature can be adjusted by
lowering or raising the steam boiler's temperature and pressure. Since
the water arriving at the group is designed to be at the correct
temperature, the group itself also has to be heated to the correct
temperature so as not to change that of the brew water. This is done
either by circulating hot water through the group or by bolting the
group directly to the boiler. Again, this is not a very precise system,
and it is difficult to adjust the temperature to a precise level.
However, good engineering can make heat exchanger systems very stable,
so that they hold the same temperature within 1°C to 2°C. This
is mainly done by using very massive groups and heat exchangers. Once
these are at the correct temperature, changes in the relatively small
amounts of water going through them do not affect their thermal
stability. However, they still depend on shots being made a steady pace.
After a long idle time, the water in the heat exchanger will overheat,
and the group may also drift to the wrong temperature. One has to go
through a regime of flushing water through the group to get the system
to the right starting temperature for making shots. The exact details of
this regime vary from machine to machine, however the article How I Stopped Worrying and Learned to Love HXs
offers general advice that can be adapted to most heat exchangers.

Heat exchanger systems have an important advantage compared to
smaller single boiler home machines. They can steam milk and make shots
at the same time, whereas single boiler machines cannot.

Single Boiler: Smaller home machines have a single boiler
without a heat exchanger. When making espresso, one thermostat is used
to heat the water to 90°C to 96°C; when steaming, another
thermostat is used to heat the water to 125°C. There can be up to a
one-minute wait for the boiler to switch from one temperature to the
other. The major quality factor in these machines is the size of the
boiler. The poorest machines have a thermoblock that heats less than an
ounce of water on the fly. The best machines have boilers up to 25 fluid
ounces. Although bigger is always better in terms of thermal stability,
above about 12 to 16 ounces of boiler size, the added stability becomes
somewhat academic compared to other factors.

In most home machines, the thermostat is a simple bi-metallic disc
mounted to the outside surface of the boiler. These have a deadband
(the range between turning on and off) of around 10°C. In order to
get consistent temperatures shot-to-shot, you begin brewing espresso at
the same point in the range, typically the moment it reaches maximum
temperature. Varying the shot temperature is very difficult, and
requires timing out the thermostat cycle precisely—a technique
called temperature surfing.

Better home machines use vapor pressure or electronic thermostats
which measure the water directly and have narrow deadbands. Although
these are adjustable in theory, in practice, they are not very
accessible.

Many home espresso enthusiasts take one of the better single boiler
home machines and use industrial temperature controllers (called PID
controllers) to precisely regulate the temperature. When this is done,
these machines deliver very repeatable and adjustable shot temperatures.

Double Boiler: Single boiler machines cannot steam and pull
shots at the same time. However, with the right controls, they deliver
very precise temperature control. Commercial manufacturers took note of
this and are producing double boiler machines. These have one boiler
used for brewing and another for steaming, with each boiler set to the
correct temperature for its function. In theory, such machines can
deliver completely stable temperatures. In practice, groups are not
regulated and shot temperatures can vary as widely as on the better heat
exchanger models. Nevertheless, since these espresso machines almost
always have accessible electronic controls, they are much easier to set
to the desired temperature than any other kind of machine. Also, this is
an active area of innovation, and double boiler espresso machines are
becoming more precise with each new model iteration. The most advanced
current models claim to keep temperatures within 0.5°C of the
setting under all operating conditions.

There are several models of double boiler machine now available for
the high end home market.

How Important Is Precision?

It depends. Most home espresso enthusiasts buy small commercial
machines, or do their own upgrades on better home models, not because
they drink more coffee, but because they are looking for consistency.
Consistency in pressure and temperature has three aspects:

Repeatability from shot to shot so that the
temperatures and pressures are the same for every espresso. This is
essential, since if the machine's pressure and temperature change from
shot to shot, you can never get a consistent taste in your espresso.
It's work to get repeatable shots on even good stock home machines.
Commercial models and DIY upgrades make this a lot easier.

Stability within the shot so that the temperature and
pressure hold the same value throughout. A machine will spoil the
espresso if the temperature or pressure vary widely. But if they vary a
small amount and in a consistent manner, there is no evidence that
espresso quality is compromised. For example, spring lever machines
deliver the same pressure curve, one that drops towards the end,
whereas motor pumps deliver even pressure throughout. HX machines
usually have "humped" temperature profiles varying a few degrees
Fahrenheit within a shot, whereas single and double boiler machines
have straight line temperatures, also varying a few degrees at most.
Repeated tests have shown no clear advantage for any of these
behaviors. It should be noted that very small pump machines that cost
in the $100 to $200 range usually are unstable, and temperatures and
pressures will vary widely and unacceptably within a shot.

Adjustability so that one can change the brew
temperature or extraction pressure. This is essential if you want to
use a particular style of blend or coffee on the machine, since
differing blends and roasts favor different pressures and temperatures.
If the machine is not easily set, one has to find a blend and roast
style that is suitable for it, and stick to it for the best espresso.
Lever machines are not easily reset. HX machines and mechanically
controlled motor pumps require opening the machine up for an
adjustment, while electronically controlled dual boiler machines, or
PIDed home machines can have their temperature and (sometimes) their
extraction pressure adjusted on the fly.

Machine Factors Not Related to Coffee Quality

You will pay more for beautifully designed cases, for higher quality,
long life components, and for good workmanship and maintainability.
Inexpensive home machines have almost become disposable, and such
features are likely a waste of money. More expensive machines are like
major appliances—their lifetime is measured in decades, and they
are designed to be repaired and serviced. Since it is more convenient to
do routine maintenance and service yourself, a well designed machine
with high quality components greatly increases the joy of ownership.
Since such machines are long term fixtures in the home, the quality and
type of case design should be chosen with their location in mind. It is
rather odd to walk into a beautiful marble tiled kitchen and see its
granite counters populated by cheap plastic gadgets; on the other hand,
a machine destined for placement behind a counter or in a working
kitchen can be simple and utilitarian in design.